A molded product of a fiber reinforcing resin in which the strength is enhanced by containing a reinforcing fiber is used for various purposes. The molded product is prepared by causing a mixture of a thermoplastic resin and the reinforcing fiber melted in response to rotations of a screw inside a cylinder configuring a plasticizing apparatus to be injected into a mold of an injection molding machine.
In order to obtain an effect of improving the strength by using reinforcing fibers, it is desired that the reinforcing fibers are uniformly dispersed in a resin. In order to realize the uniform dispersion, a shearing force applied to the reinforcing fibers may be strengthened under strict conditions of mixing. However, an excessively strong shearing force results in cutting of the reinforcing fiber. Consequently, the fiber length after molding becomes significantly shorter than the initial fiber length, and there is a possibility that an obtained molded product cannot satisfy desired characteristics.
As a countermeasure thereof, PTL 1 discloses that a molded product in which comparatively long fibers are homogenously dispersed can be obtained by using a vent-type injection molding machine in which vent-up is unlikely to occur, feeing the reinforcing fibers through a vent port, and mixing the reinforcing fibers with a molten resin from the upstream side. PTL 1 proposes a specific injection molding machine including a raw material feed zone, a first stage, a mixing (kneading) portion, and a second stage. In the injection molding machine, a vent portion is provided immediately after the mixing portion, and the second stage has a shape of a multi-start screw including three or more threads.